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Four electric charges +q, +q, –q and –q are placed at the corners of a square of side 2L (see figure). The electric potential at point A, midway between the two charges +q and +q, is

1. zero
2. 4q / (4π ε₀ L)
3. 8q / (4π ε₀ L)
4. 16q / (4π ε₀ L)

Correct answer: 4q / (4π ε₀ L)

Solution

The potential at point A is the algebraic sum of potentials due to all four charges. The contributions from the two +q charges add up, while the contributions from the two -q charges cancel out due to symmetry. The result is V = 4q / (4π ε₀ L).

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